clk: move checking .get_parent to __clk_core_init()

The .get_parent is mandatory for multi-parent clocks.  Move the check
to __clk_core_init(), like other callback checkings.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
[sboyd@codeaurora.org: Squashed in error path handling, fix typos
in commit message]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>